Subject: Re: [PATCH RFC 1/3] drivers: base: support cpu cache information interface to userspace via sysfs

From: Greg Kroah-Hartman <[email-masked]>
Date: Thu, 9 Jan 2014 11:31:21 -0800

On Thu, Jan 09, 2014 at 07:19:00PM +0000, Sudeep Holla wrote:
> On 08/01/14 20:27, Greg Kroah-Hartman wrote:
> > On Wed, Jan 08, 2014 at 07:26:06PM +0000, Sudeep Holla wrote:
> >> From: Sudeep Holla <[email-masked]>
> >>
> >> This patch adds initial support for providing processor cache information
> >> to userspace through sysfs interface. This is based on x86 implementation
> >> and hence the interface is intended to be fully compatible.
> >>
> >> A per-cpu array of cache information maintained is used mainly for
> >> sysfs-related book keeping.
> >>
> >> Signed-off-by: Sudeep Holla <[email-masked]>
> >> ---
> >>  drivers/base/Makefile     |   2 +-
> >>  drivers/base/cacheinfo.c  | 296 ++++++++++++++++++++++++++++++++++++++++++++++
> >>  include/linux/cacheinfo.h |  43 +++++++
> >>  3 files changed, 340 insertions(+), 1 deletion(-)
> >>  create mode 100644 drivers/base/cacheinfo.c
> >>  create mode 100644 include/linux/cacheinfo.h
> >>
> >> diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> >> index 94e8a80..76f07c8 100644
> >> --- a/drivers/base/Makefile
> >> +++ b/drivers/base/Makefile
> >> @@ -4,7 +4,7 @@ obj-y			:= core.o bus.o dd.o syscore.o \
> >>  			   driver.o class.o platform.o \
> >>  			   cpu.o firmware.o init.o map.o devres.o \
> >>  			   attribute_container.o transport_class.o \
> >> -			   topology.o
> >> +			   topology.o cacheinfo.o
> >>  obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
> >>  obj-$(CONFIG_DMA_CMA) += dma-contiguous.o
> >>  obj-y			+= power/
> >> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> >> new file mode 100644
> >> index 0000000..f436c31
> >> --- /dev/null
> >> +++ b/drivers/base/cacheinfo.c
> >> @@ -0,0 +1,296 @@
> >> +/*
> >> + * cacheinfo support - processor cache information via sysfs
> >> + *
> >> + * Copyright (C) 2013 ARM Ltd.
> >> + * All Rights Reserved
> >> + *
> >> + * Author: Sudeep Holla <[email-masked]>
> >> + *
> >> + * This program is free software; you can redistribute it and/or modify
> >> + * it under the terms of the GNU General Public License version 2 as
> >> + * published by the Free Software Foundation.
> >> + *
> >> + * This program is distributed "as is" WITHOUT ANY WARRANTY of any
> >> + * kind, whether express or implied; without even the implied warranty
> >> + * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >> + * GNU General Public License for more details.
> >> + */
> >> +#include <linux/bitops.h>
> >> +#include <linux/cacheinfo.h>
> >> +#include <linux/compiler.h>
> >> +#include <linux/cpu.h>
> >> +#include <linux/device.h>
> >> +#include <linux/init.h>
> >> +#include <linux/kobject.h>
> >> +#include <linux/of.h>
> >> +#include <linux/sched.h>
> >> +#include <linux/slab.h>
> >> +#include <linux/smp.h>
> >> +#include <linux/sysfs.h>
> >> +
> >> +struct cache_attr {
> >> +	struct attribute attr;
> >> +	 ssize_t(*show) (unsigned int, unsigned short, char *);
> >> +	 ssize_t(*store) (unsigned int, unsigned short, const char *, size_t);
> >> +};
> >> +
> >> +/* pointer to kobject for cpuX/cache */
> >> +static DEFINE_PER_CPU(struct kobject *, ci_cache_kobject);
> >> +#define per_cpu_cache_kobject(cpu)     (per_cpu(ci_cache_kobject, cpu))
> >> +
> >> +struct index_kobject {
> >> +	struct kobject kobj;
> >> +	unsigned int cpu;
> >> +	unsigned short index;
> >> +};
> >> +
> >> +static cpumask_t cache_dev_map;
> >> +
> >> +/* pointer to array of kobjects for cpuX/cache/indexY */
> > 
> > Please don't use "raw" kobjects for this, use the device attribute
> > groups, that's what they are there for.  Bonus is that your code should
> > get a lot simpler when you do that.
> > 
> 
> Yes I now understand device attribute group simplifies the code, but I think
> kobjects are still needed as we need to track both cpu and cache index.
> By reusing only cpu device kobject, we can track cpu only.

I don't understand, you are putting things under the cpu device object,
why do you care about a "cache" kobject?

> One thought I have is to make cache_info structure common to all architecture
> (for now its ARM specific) and introduce kobject in that similar to ia64
> implementation. That even eliminates lot of weak functions defined.

Please don't use raw kobjects if at all possible, it's not good for a
variety of reasons (no userspace events, have to roll your own code,
etc.)

thanks,

greg k-h